1. Field of the Invention
The present invention relates to a protective film forming composition used in a manufacturing process of semiconductor devices, such as IC, manufacture of circuit substrates for liquid crystals, thermal heads and the like, and lithographic process of other photo-fabrication, and also the invention relates to a pattern-forming process using the composition. In particular, the invention relates to a protective film-forming composition suitable for exposure with an immersion projection exposure apparatus using far ultraviolet rays of 300 nm or less as the light source, and a pattern-forming process using the composition.
2. Description of the Related Art
With the progress of fining of semiconductor elements, shortening of the wavelengths of exposure light source and increasing of the numerical aperture (high NA) of projection lens have advanced, and now exposure apparatus of NA 0.84 using an ArF excimer laser having the wavelength of 193 nm as the light source have been developed. As generally known, these can be expressed by the following equations:(Resolution)=k1·(λ/NA)(Depth of focus)=±k2·λ/NA2 wherein λ is the wavelength of exposure light source, NA is the numerical aperture of the projection lens, k1 and k2 are the coefficients concerning the process.
For further higher resolution by the shortening of wavelengths, an exposure apparatus with an F2 excimer laser having the wavelength of 157 nm as the light source has been studied, but the materials of lens for use in the exposure apparatus for shortening of wavelengths and the materials of resist are extremely restricted, so that the realization of the reasonable manufacturing costs of the apparatus and materials and quality stabilization are very difficult, as a result, there are possibilities of missing an exposure apparatus and a resist having sufficient performances and stabilities within a required period of time.
As a technique for increasing resolution in optical microscopes, a so-called immersion method of filling a liquid of high refractive index (hereinafter also referred to as “immersion liquid”) between a projection lens and a sample has been conventionally known.
As “the effect of immersion”, the above resolution and depth of focus can be expressed by the following equations in the case of immersion, taking λ0 as the wavelength of the exposure light in the air, n as the refractive index of immersion liquid to the air, and NA0=sin θ with θ as convergence half angle of the ray of light:(Resolution)=k1·(λ0/n)/NA0 (Depth of focus)=±k2·(λ0/n)/NA02 
That is, the effect of immersion is equivalent to the case of using exposure wavelength of the wavelength of 1/n. In other words, in the case of the projection optical system of the same NA, the depth of focus can be made n magnifications by immersion.
This is effective for every pattern form, further, this can be combined with super resolution techniques such as a phase shift method and a deformation lighting method now under discussion.
As the example of apparatus applying this effect to the transfer of micro-fine image pattern of semiconductor element, JP-A-57-153433 and JP-A-7-220990 are known, but resists suitable for immersion exposure techniques are not disclosed in these patents.
It is appointed in JP-A-10-303114 that the control of the refractive index of an immersion liquid is important as the variation of the refractive index of an immersion liquid causes the deterioration of a projected image due to the wave surface aberration of exposure apparatus, and controlling the temperature coefficient of the refractive index of an immersion liquid to a certain range, and water added with additives for reducing surface tension or increasing the degree of surface activity are disclosed as a preferred immersion liquid. However, the specific additives are not disclosed and resists suitable for the technique of immersion exposure are not also discussed.
The latest technical progress of immersion exposure is reported in SPIE Proc., 4688, 11 (2002), and J. Vac. Sci. Tecnol. B, 17 (1999). When an ArF excimer laser is used as the light source, it is thought that pure water (refractive index of 1.44 at 193 nm) is most promising in the light of the safety in handling, the transmittance and the refractive index at 193 nm.
When an F2 excimer laser is used as the light source, a solution containing fluorine is discussed from the balance of the transmittance and the refractive index at 157 nm, but a sufficiently satisfactory solution from the viewpoint of the environmental safety and at the point of refractive index has not been found yet. From the extent of the effect of immersion and the degree of completion of resist, it is thought that immersion exposure technique will be carried on an ArF exposure apparatus earliest.
From the advent of the resist for a KrF excimer laser (248 nm) on, an image-forming method that is called chemical amplification is used as the image-forming method of the resist for compensating for the reduction of sensitivity by light absorption. To explain the image-forming method of positive chemical amplification by example, this is an image-forming method of exposing a resist to decompose an acid generator in the exposed area to thereby generate an acid, utilizing the generated acid as the reactive catalyst to change an alkali-insoluble group to an alkali-soluble group by the bake after exposure (PEB: Post Exposure Bake), and removing the exposed area by alkali development.
In immersion exposure, a resist film is exposed through a photomask in the state of filling an immersion liquid between the resist film and the optical lens, to transfer the pattern of the photomask to the resist film. At this time, there are cases where an image is not formed by the osmosis of an immersion liquid into the inside of a resist film (Nikkei Micro-device, April, 2004). Further, it is imagined that organic substances and the like are eluted from a resist film and get into an immersion liquid as impurities and contaminate a lens and an exposure apparatus to thereby hinder exposure.
As a solution to avoid such problems, a method of providing a protective film between a resist film and a lens (hereinafter referred to as “a topcoat” or “an overcoat”) so that a resist and water do not come in contact directly is known (e.g., Nikkei Micro-device, April, 2004).
However, the materials having sufficiently satisfactory performances for use as the topcoat are not found yet.